1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, to an imaging apparatus including a print media feed system configured for reducing printing defects.
2. Description of the Related Art
A typical ink jet printer forms an image on a print medium by ejecting ink from a plurality of ink jetting nozzles of an ink jet printhead to form a pattern of ink dots on the print medium. Such an ink jet printer typically includes a reciprocating printhead carrier that transports one or more ink jet printheads across the print medium along a bi-directional scanning path defining a print zone of the printer. Typically, the mid-frame provides media support at or near the print zone. A sheet feeding mechanism is used to incrementally advance the print medium sheet in a sheet feed direction, also commonly referred to as a sub-scan direction or vertical direction, through the print zone between scans in the main scan direction, or after all data intended to be printed with the print medium at a particular stationary position has been completed.
One such sheet feed mechanism includes a feed roller and corresponding pinch roller arrangement located upstream of the print zone, and an exit roller and corresponding exit pinch roller arrangement, such as a plurality of star wheels, located downstream of the print zone. The exit roller may be, either intentionally or unintentionally, slightly over-driven to place the sheet in a state of slight tension during printing. Such a sheet feed mechanism, however, does not easily permit printing near the trailing edge of the sheet, as in attempting borderless printing, since as the sheet is released from the feed roller, the sheet can lunge forward due to the state of tension of the sheet and/or the allowable play or backlash in the gear train of the sheet feed mechanism, thereby resulting in a printing defect referred to as horizontal banding. As the name implies, horizontal banding is a horizontal band across the width of the sheet of print media where a uniform swath wide dot placement error occurs due to media indexing inaccuracies. Thus, on a sheet of print media, if during printing the sheet indexes inaccurately as the sheet is released from the feed roller nip, an undesirable horizontal band will appear on the sheet.
One known gear drive system for improving the accuracy and control of media advancement and positioning includes a friction device to keep the teeth of the respective drive (feed roller) and tension (exit roller) gears meshed together with the teeth of the corresponding pinion gears of the motor, even if the motor backs up slightly. The friction device includes elements that pinch the gears of the gear train, adding friction so that when the motor stops and backs up, the gears follow it backwards, thereby keeping the gear teeth meshed together. However, such a system creates undesirable drag on the gear train, thereby increasing motor torque and, in turn, increasing the electrical energy requirements for the gear drive system.
What is needed in the art is a print media feed system that permits precise control of the position of a sheet of print media following release by the feed roller without introducing undesirable drag on the gear train.